1. Field of the Invention
This invention relates generally to the field of assessing bone mineral density, and more particularly to assessing oral bone mineral density using a panoramic x-ray system and a reference phantom.
2. Related Art
Bone mineral density (BMD, g/cm2) is widely used by doctors and physicians in detecting and managing osteoporosis and in predicting fracture, particularly in higher-risk patients such as postmenopausal women. Osteoporosis is a disease in which the bones become extremely porous and fragile, are subject to fracture, and heal slowly and often poorly. This disease occurs especially in women following menopause and often leads to curvature of the spine from vertebral collapse. The thinning of the bones with a reduction in bone mass is generally due to a depletion of calcium. Left unchecked, osteoporosis can lead to changes in posture, physical abnormality (for example, a hunched back), and decreased mobility. Early diagnosis of osteoporosis is critical and treatments have been shown to slow progressive bone loss and reduce the risk of fracture. Such treatments include calcium supplementation, hormone replacement therapy, and the use of bisphosphonates (a class of drugs used to strengthen bone) and intermittent parathyroid hormone (a hormone critical to calcium and phosphorus balance).
BMD is believed to be the single best predictor of fracture in asymptomatic postmenopausal women. (See, e.g., J. A. Kanis et al., “The Diagnosis of Osteoporosis,” J Bone and Mineral Res. 1994; 9:1137-1141.) While densitometry of the hip and spine has become the so-called “gold-standard” for assessing the risk of fracture, a variety of peripheral sites, such as the finger, are also utilized and have been found to be predictive of future fracture in large prospective trials. See, e.g., S. Miller et al., “Identification and Fracture Outcomes of Undiagnosed Low Bone Mineral Density in Postmenopausal Women,” JAMA 2001; 286(22): 2815-2821; also see, e.g., U.S. Pat. No. 5,852,647 (Schick, et al.), U.S. Pat. No. 5,898,753 (Schick, et. al.), U.S. Pat. No. 5,912,942 (Schick, et al.), and U.S. Pat. No. 6,069,935 (Schick, et al.).
Measuring BMD may be particularly interesting to practitioners in the dental industry for a variety of reasons. First, similar to other peripheral sites, it appears that oral BMD is related to systemic BMD, and therefore can also be a reliable predictor of osteoporosis. Dentists tend to see their patients often, typically performing x-ray examinations every 6-12 months, which is a greater regularity than that with which general and other physicians see many of their patients. The dentists' office, therefore, can be an excellent place for routine BMD tests to occur, and there is a need for an in-office tool that a dentist can use to take such tests.
Furthermore, a measurement of oral BMD may prove beneficial in the diagnosis of periodontal disease, which is a bacterial infection that can destroy the attachment fibers and supporting bone that hold the teeth in the mouth. Such a bacterial infection typically causes the loss of soft tissue attachment and the resorption of alveolar bone (the jaw section containing the tooth sockets). Resorption is the process of losing substance, as bone, when it is remodeled or reshaped, undergoes both new formation and resorption. The loss of soft tissue attachment and the resorption of alveolar bone may result in tooth loss, edentulousness, and resorption of the remaining residual ridge, which in turn can lead to a further loss of oral bone.
There is therefore a great need for an in-office tool for measuring oral BMD so that dental practitioners such as periodontists, who are concerned with prevention, diagnosis, and treatment of diseases affecting the gums and supporting structures of the teeth, could potentially stage the progression of disease in the mouth and make better treatment decisions.
BMD measurements are also commonly used in connection with tooth implant surgery. In particular, tooth implant surgery is commonly preceded by a measure of BMD by quantitative computed tomography (described in more detail below) or cone beam tomography to ensure that the mouth has sufficient integrity to support the operation. Of course, BMD measurements are useful in other applications as well.
Conventional central and peripheral densitometry techniques are understood by those having ordinary skill in the art. Most commercially available methods for BMD measurement pass a low-intensity beam of x-rays through a patient, and a radiation detector on the other side measures how much of the beam is absorbed. Part of the beam is absorbed by the bone and part by the surrounding soft tissue, and each technique measures these differently. Generally, such techniques involve an energy source of either single or dual energy. Whereas early densitometry devices utilized radiologic isotopes as the energy source, over time these were replaced with more stable and sophisticated x-ray generators.
Known techniques include single energy x-ray absorptiometry (SXA), radiographic absorptiometry (RA), computed tomography (CT), quantitative computed tomography (QCT), and dual-energy x-ray absorptometry (DEXA). SXA computes bone mineral from the increased absorption of the beam as it passes through soft tissue and bone. RA measures BMD in extremities such as the fingers relative to an aluminum calibration wedge on the film. CT utilizes an array of detectors to collect x-ray attenuation data from x-rays that pass through the body. The data are input as digital data into a computer, which processes that data and reconstructs planar cross-sectional images of the internal structures of the body through which x-rays pass. QCT requires the use of a bone-equivalent calibration phantom which is scanned simultaneously with the patient to provide BMD measurements in axial images. The planar images can be further reconstructed into three-dimensional voxels and QCT is the only known technique for computing true three-dimensional density. DEXA uses a dual energy approach, and in particular, uses two planar x-ray images obtained using x-rays with different energy levels to compensate for tissue variations and quantify bone mass in an x-ray image. Typical existing DEXA systems rely on known x-ray absorption characteristics of hard tissue and soft tissue to both high-energy and low-energy radiation, thereby enabling existing DEXA systems to determine the hard tissue mass.
Densitometry techniques known in the art include both film and digital-based techniques. Generally, an x-ray filter separates the energy spectra and a reference standard is included to correlate x-ray attenuation to BMD. An example is provided in U.S. Pat. No. 5,335,260 (Arnold), which utilizes a calibration phantom representative of human tissue containing variable concentrations of calcium. The calibration phantom serves as a calibration reference for quantifying calcium, bone mass and bone mineral density in radiography and CT imaging systems.
The general principles discussed above have been applied to measure a variety of anatomic sites, including oral structures. U.S. Patent Application Publication No. US 2002/0114425 A1 (Lang et al.), for example, discusses the measurement of oral BMD from a dental x-ray image which includes using a calibration phantom and a software algorithm to relate pixel intensity to measures of bone mineral density and/or architecture.
Dental imaging can be generally segmented into either intra-oral or extra-oral modalities. In intra-oral imaging, an electronic sensor is located inside the mouth, whereas in extra-oral imaging, the sensor is located outside the mouth. Intra-oral techniques are generally less ideal for densitometry, since the so-called source-to-target distance is not typically fixed. Moreover, in intra-oral imaging, the dental x-ray tube generally maintains the freedom to rotate in multiple planes, which may lead to an inaccurate or imprecise assessment. Further, fashioning a reference phantom small enough to be suitable for the mouth is challenging. Although there may be ways of attempting to control these variables, utilizing this technique in a clinical environment may be challenging and therefore it is preferable to altogether remove these potential error sources.
Extra-oral examinations may be better suited for densitometric assessment since, with the detector outside the mouth, the evaluation is more controllable and may be more comfortable for the patient. In particular, whereas in intra-oral imaging a reference standard would need to be included in the mouth and over the detector, in extra-oral imaging there would be more flexibility of placement. Furthermore, in extra-oral imaging the clinician will generally have more freedom to analyze the particular anatomic region of interest. U.S. Pat. No. 6,381,301 (Massie) discusses a tomographic system for determining bone mineral density of a patient's mouth. Unfortunately, a drawback to this system is that few dentists have systems with tomographic capability within their practice, and therefore this technique does not provide a practical commercial tool. Standard panoramic dental x-ray systems are much more common and allow a complete view of the patient's anatomy. Panoramic measurements of mandibular inferior cortical shape and width, for instance, have been proposed as a potentially useful tool for screening spinal osteoporosis. These geometric measures are, however, an indirect index of bone mineral density.
There exists therefore a need for a fresh approach which provides a more practical tool for enabling dental practitioners to assess BMD for the purpose of treating their patients. Such a tool should be flexible, easy to use, and should capitalize on equipment that is typically found in most dental offices.